1. Field of the Invention
This invention relates to the field of data processing, and, more particularly, to a method and apparatus for improving the demarcation between different areas of a derivative bit mapped image using a limited number of colors or dither patterns, which image is derived from a primary bit mapped image having a greater number of colors.
2. Description of Related Art
Often, a person using a personal computer having a color monitor desires a printed copy of the image or screen being displayed on the monitor. The results of printing the screen are often poorly demarcated and illegible, and look as if someone rolled a paint roller dipped in black paint over the sheet. This anomaly is attributable to the nature of commonly used monitors and printers. Displayed images normally have many colors. For example, the standard IBM color graphics array (CGA) can display 16 colors while the standard IBM video graphics array (VGA) can display 256 colors. On the other hand, the vast majority of printers are concerned with only two colors, the white of the paper and the black of the ink.
The most common techniques for attacking this problem are dithering and cross hatching. "Dithering" is the term used hereafter to refer to both techniques. Dithering uses various pixel patterns to demarcate areas in a derivative bitmap. For example, two adjacent areas (one a purple area and one a red area) in a primary bitmap may be rendered in a derivative bitmap using solid black for one area and a checkerboard pattern in the other area.
The limitations of dithering become more pronounced as the resolution or number of colors in the primary bitmap increases, or as the resolution or number of colors in the derivative bitmap decreases. Many printers do not have substantially higher resolution than the display being printed, and this makes it difficult to come up with a large number of visually unique dither patterns.
Dithering algorithms commonly attempt to map each unique color in the primary bitmap to a unique color/dither pattern in the derivative bitmap. The algorithms try to produce a 1:1 correspondence between the colors of the primary bit map and the colors/dither patterns of the derivative bitmap. They also maximize the number of unique color/dither patters in the derivative image but reduce demarcation and legibility. This result is unfortunate because, very often, a user is more interested in clearness and demarcation of the derivative (printed) bitmap than in preserving a 1:1 correspondence or mapping between different areas of the primary and derivative bitmaps.